Much of modern technology makes use of thin solid films on the surfaces of solid substrates. A number of methods have been used to deposit such thin films including thermal evaporation, DC sputtering, rf sputtering, ion beam deposition, chemical vapor deposition, plating, molecular beam deposition and deposition from the liquid phase.
The structure of thin films can be amorphous (that is, the atoms of the film are not arranged in any crystalling order), polycrystalline (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, but the small regions have no mutual alignment of their crystallographic axes), preferred orientation (that is, the film is composed of many small regions, in each of which the atoms are arranged in a regular crystalline order, and one or more of the crystalline axes of the majority of said regions are parallel), or epitaxial (that is, the film is predominantly of a single crystallographic orientation). An epitaxial or single crystal film is a special case of a preferred orientation film in which corresponding crystallographic axes of all the small regions are essentially oriented in the same directions. A thin film can be the same material (that is, the same element or compound) as the substrate, or it can differ in chemical composition from the substrate. If the film is epitaxial, the former is called "homoepitaxy" and the latter "heteroepitaxy".
In many solid state electronic devices, the active volume of the device comprises or lies within a thin sheet, film or layer of crystalline semiconductor material, preferably in the single crystal or monocrystalline form. This active volume is formed on an insulative substrate. This is particularly true of integrated circuits formed from semiconductors such as gallium arsenide, silicon, germanium, indium phosphide, cadmium telluride, etc. Present techniques for fabricating such devices, however, require that the crystalline sheets or films be grown mainly by chemical vapor deposition from the surface of relatively thick substrates of high-purity, single crystal semiconductor material, such as sapphire. The use of such substrates for each sheet produced tends to inordinately increase the cost of producing the thin sheets. Furthermore, the high defect density of the epitaxial sheets or films thus formed and the high dielectric constant of the sapphire limits the performance of the resulting device.
Another alternative has been to form large grain sheets or films using a scanned laser beam to heat an amorphous or polycrystalline sheet or film of semiconductor material, such as Si, which has been deposited on sapphire or fused silica (SiO.sub.2). See, for example, U.S. Pat. No. 4,059,461. Small grains, less than 50 microns in size, are obtained by this process and the film when overlayed on SiO.sub.2 has a tendency to crack. When processed on sapphire, the Si film may interact detrimentally with the sapphire.
Another recent innovation in the growth of epitaxial films is described in a paper entitled "Silicon Graphoepitaxy Using a Strip-Heater Oven" by M. W. Geis, D. A. Antoniadis, D. J. Silversmith, R. W. Mountain and Henry I. Smith. Appl. Phys. Lett. 37(5) Sept. 1, 1980 which relates to the conversion of amorphous silicon to a silicon mosaic film by graphoepitaxy with a film or "cap" of SiO.sub.2 over the silicon film. This process is also described in copending U.S. patent application Ser. No. 181,102 filed Aug. 25, 1980 and entitled "Graphoepitaxy by Encapsulation" (now abandoned), continuation Ser. No. 332,553 filed Dec. 21, 1981 (now abandoned), continuation Ser. No. 680,237 filed Dec. 10, 1984 (now U.S. Pat. No. 4,565,599).
The graphoepitaxy process requires the intentional creation of artificial surface relief steps or point defects in a geometric pattern on a substrate to control in a predetermined way the process of film formation and growth on the substrate. The geometric pattern is generally a simple grating or grid, oriented to promote crystal growth in a predetermined manner.
In the graphoepitaxy process, the substrate crystal orientation plays a passive role in the formation of the crystal orientation and growth which is determined primarily by the surface relief structure.
As reported on page 455 of the referenced Appl. Phys. Lett. even with a "cap", if the silicon film is fully melted in a graphoepitaxial process, neither texture or orientation of the crystallized film is observed. Apparently, the function of the SiO.sub.2 cap in the graphoepitaxy process is to produce a shear stress as a result of thermal expansion differences between Si and SiO.sub.2 which leads to anisotropy in the Si such that upon crystallization a (100) texture and uniform orientation relative to the relief gratings occurs.
Also, recently in copending U.S. patent application Ser. No. 138,891 filed Apr. 10, 1980 (now abandoned) and incorporated in continuation-in-part application Ser. No. 251,214 filed Apr. 6, 1981, there is disclosed a method of achieving lateral epitaxial growth by seeded solidification (melting and refreezing of amorphous Si) through openings in an insulator formed on a crystallization substrate. After the growth is discontinued, the sheet of crystalline material is cleaved or otherwise separated from its substrate which may optionally be reused. This process we shall refer to for the sake of convenience as the CLEFT process and the application, and the CLEFT application.
While the CLEFT process is believed to represent a significant advance over the state-of-the-art at the time the invention was made; certain problems have arisen in the application of the CLEFT invention to the consistent production of high quality, defect free, epitaxial films. More specifically, if for any reason, the lateral epitaxial growth is discontinuous, then further epitaxial growth is precluded since the further growth has no crystalline orientation to start from and will therefore grow in random polycrystalline fashion. Accordingly, a need exists for a method and apparatus for minimizing discontinuities in epitaxial films laterally grown through openings in an insulative mask formed on a crystallization substrate.
We have also found that where lateral growths meet, i.e., when lateral growth in one direction encounters lateral growth from another direction, dislocation defects are apt to occur; presumably caused by the strain or stress created when two crystallization fronts meet.
For these and other reasons, it would be highly desirable to have a process and apparatus for producing lateral epitaxial growth in amorphous semiconductor material wherein discontinuities in growth and crystal dislocations are minimized and which is relatively simple in operation and low in cost.